Nanoparticles are particles having nanoscale particle sizes, and show optical, electrical and magnetic properties completely different from those of bulk materials due to large specific surface area and the quantum confinement effect, in which energy required for electron transfer changes depending on the size of material. Due to such properties, much interest has been concentrated on their applicability in the catalytic, electromagnetic, optical and medical fields. Since nanoparticles can be considered as intermediates between bulk materials and molecules, they can be synthesized using two approaches, i.e., the “top-down” approach and the “bottom-up” approach. The “top-down” approach is a method of breaking bulk material into small pieces, and has an advantage in that it is easy to control the size of nanoparticles, but a disadvantage in that it is difficult to make less than 50 nm of nanoparticles. For this reason, the “bottom-up” approach, i.e. a method of assembling atoms or molecules into nanoparticles, has recently received attention, and is mainly performed using colloidal synthesis starting from chemical molecular or atomic precursors.
With regard to the synthesis of metal nanoparticles, the synthesis of metal nanoparticles, including gold, silver, platinum, palladium, ruthenium, iron, copper, cobalt, cadmium, nickel and silicon, has been reported. However, because such metal nanoparticles are unstable by themselves, they aggregate with the passage of time and thus lose their nanoparticle properties. Thus, for the synthesis of nanoparticles, which are stable in solution and even after drying, a method capable of preventing these nanoparticles from aggregating together and a method capable of preventing the surface of the nanoparticles from being oxidized are required.
Meanwhile, core-shell type nanoparticles, which comprise nanoparticle as the core and another material coated on the surface of the core, are known in the art. In the core-shell type nanoparticles, the shell portion forms a chemical and mechanical protective layer for the core material, and the material of the core and the material of the shell provide multi-functionality while maintaining the inherent property of each thereof, or the two properties will interact with each other to show new properties. Thus, the core-shell type nanoparticles are applicable in various fields, including catalyst and photoelectric device fields. However, it is not easy to embody multi-layer structures at the nanoscale.
For example, such core-shell nanoparticles are used as precursors either for preparing hollow structures by completely removing the core material through chemical etching, combustion, photodissolution and the like, or for preparing a unique structure of materials by partially removing the core material. Also, there is an example in which TiO2, CeO2 or the like, having photocatalytic functionality, is used as the core material, and a metal such as Ag or Cu, having antibacterial activity, is coated on the surface of the core material. Other examples, in which a metal is used as the core material, and inorganic metal oxide is coated as the shell material, include a case in which SiO2 is coated on magnetic material Ni to provide chemical and magnetic stabilities, and a case in which SiO2 is coated on metal nanoparticles such as Au or Ag to provide chemical stability. Particularly, with regard to the case where SiO2 is coated as the shell material, it is known to coat SiO2 using, for example, TEOS (TetraEthyl OrthoSilicate), an organometallic compound of silicon, as a precursor, through a hydrolysis-condensation reaction. However, when this reaction is used to prepare the metal core-oxide shell nanoparticles, there are problems in that oxide coated on the surface of the core is amorphous in nature, and thus has reduced chemical and mechanical properties compared to crystalline oxide.